In today's society, there are many conflicting social goals that can rarely be reconciled. Specifically, safety and noise pollution frequently come into conflict. Emergency vehicle sirens and train whistles are examples of such diverse goals. While it is clearly necessary that pedestrians and drivers are audibly alerted to the approach of one of these vehicles, these sirens and whistles have had to become louder to be heard over the background noise of traffic; and thus these devices also are clearly heard by others who need not be aware of the presence of the vehicle. For purposes of describing this invention, trains and train crossings will be used as an example, however, the invention is much broader than this area, as will be apparent to one skilled in the art after reading this specification.
As a train approaches a grade crossing (railroad tracks intersecting a road at the same level), the engineer is required by law to give four whistle blasts to alert motorists on the road of its approach. Whistle is used herein to mean horn or other warning device. People living and working near the grade crossing are often disturbed by the loudness of these blasts, especially at night. New, tougher safety rules mandate minimum loudness levels that must be used even at night, causing political backlash from citizens living near the tracks. The sound energy generated by the standard air horns disperses in a roughly hemispherical pattern, so much of the energy is wasted on regions that are nowhere near the grade crossing area that the alert is intended for.